Define bridge in MOIB

[blegat]

Discussed in #320 (comment)
There is a slight increase in loading time but it does not seem significant. As JuMP might define these bridges anyway, it should affect JuMP's loading time.

Before this PR:

$ julia
julia> @time using MathOptInterface
  0.547734 seconds (460.14 k allocations: 25.334 MiB, 10.26% gc time)
$ julia
julia> @time using MathOptInterface
  0.548062 seconds (460.14 k allocations: 25.335 MiB, 9.92% gc time)

After this PR:

$ julia
julia> @time using MathOptInterface
  0.593794 seconds (513.22 k allocations: 27.837 MiB, 9.64% gc time)
$ julia
julia> @time using MathOptInterface
  0.591734 seconds (513.22 k allocations: 27.837 MiB, 9.62% gc time)

[codecov-io]

Codecov Report

Merging #325 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #325      +/-   ##
==========================================
- Coverage   96.48%   96.45%   -0.04%     
==========================================
  Files          30       30              
  Lines        4525     4514      -11     
==========================================
- Hits         4366     4354      -12     
- Misses        159      160       +1

Impacted Files	Coverage Δ
src/Bridges/bridge.jl	98.61% <0%> (-1.39%)	⬇️

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[mlubin]

This is a big improvement for usability.